Democratic Taiwan lives under the constant threat of invasion by China, which sees the self-ruled island as part of its territory to be seized one day.

Beijing has ramped up military, diplomatic and economic pressure since the 2016 election of Taiwanese President Tsai Ing-wen, who rejects its stance that the island is part of China.

In a daily update on Tuesday morning Taiwan's defence ministry said 21 aircraft entered the island's southwest air defence identification zone (ADIZ) over the last 24 hours, including 18 nuclear-capable H-6 bombers.

That represents by far the largest daily sortie by H-6 bombers since Taipei first began releasing daily incursion data in September 2020, according to a database maintained by AFP.

The wave came after China slapped fresh import bans on Taiwanese food, beverages, alcohol and fishery products last week, prompting Premier Su Tseng-chang to accuse Beijing of violating international trade rules and "discriminating" against the island.

The H-6 is China's main long distance bomber and is able to carry nuclear payloads.

It is rare for China to send more than five H-6 bombers in one day. But sorties have increased dramatically in recent weeks.

Until recently October 2021 was the month with the largest number of H-6 flights on record at 16.

But last month China sent 21 of the bombers into Taiwan's ADIZ. And the current tally for December stands at 23.

Many nations maintain air defence identification zones (ADIZ), including the United States, Canada, South Korea, Japan and China.

They are not the same as a country's airspace.

Instead, they encompass a much wider area, in which any foreign aircraft is expected to announce itself to local aviation authorities.

Taiwan's ADIZ is much larger than its airspace. It overlaps with part of China's ADIZ and even includes some of the mainland.